Monolithic poly(methylsilsesquioxane) aerogels with glasslike transparency: From sol-gel synthesis to ambient pressure drying

doi:10.21203/rs.3.rs-4871964/v1

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Monolithic poly(methylsilsesquioxane) aerogels with glasslike transparency: From sol-gel synthesis to ambient pressure drying

https://doi.org/10.21203/rs.3.rs-4871964/v1

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published 29 Aug, 2024

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Practical aspects of the successful preparation of monolithic poly(methylsilsesquioxane) (PMSQ) aerogels with glasslike transparency via ambient pressure drying (APD) are discussed in detail. Two-step acid-base process starting from methyltrimethoxysilane (MTMS) in the presence of nonionic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) surfactant and the use of strong base as polycondensation catalyst resulted in fine mesoporous structure, showing low density (0.148 g cm⁻³) and glasslike transparency (95% at 10 mm thickness). Cracking and irreversible shrinkage during APD have been prevented by optimized aging and drying processes. In particular, aging in an aqueous alcohol solution containing a low concentration of MTMS under controlled temperature has been found to be crucial in obtaining PMSQ aerogels with crack-free, low-density, glasslike transparency, and monolithic nature. A large-area APD aerogel in 120×120×6 mm³, with thermal conductivity of 15.6 mW m⁻¹ K⁻¹, has successfully been obtained due to optimizations of aging and drying conditions. Similar APD aerogels have also been obtained when alkali metal hydroxides, especially lithium hydroxide, are employed as base catalysts. These findings are expected to play important roles in designing industrial productions of monolithic aerogels for thermal superinsulation and other applications.

Aerogel

monolith

ambient pressure drying

structural control

poly(methylsilsesquioxane)

Poly(methylsilsesquioxane) aerogels with glasslike transparency were prepared from methyltrimethoxysilane.
Aerogels were obtained also from ambient pressure drying in addition to supercritical drying.
A large-area aerogel monolith with low thermal conductivity was successfully obtained.

Monolithic gels and glasses from the sol-gel process have been attracting fundamental and technological interests since the dawn of sol-gel research [1–4]. Late Emeritus Professor Sakka was one of the foresighted and significant researchers in pursuing monolithic sol-gel glasses as alternative materials from the conventional melting method. Large-volume monolithic silica gels from optimized starting composition and synthetic conditions were prepared with the aid of drying control chemical additives (DCCAs) employed in the starting solutions [5]. The DCCAs such as formamide [6] and N,N-dimethylformamide [5] with low vapor pressure remained in the final stage of drying, reducing the surface tension of the drying solvent, which allowed crack-free drying of monolithic silica. Although these materials were not industrialized on a large scale, significant progress has been made in drying brittle silica gels prepared in the liquid phase. Kajihara et al. later prepared silica glass monoliths from sol-gel-derived macroporous silica gel monoliths via simple sintering processes [7].

Nearly a half-century before the enthusiasm for silica gel/glass monoliths, drying utilizing supercritical fluid was reported by Kistler, in which various materials, including silica gel, were dried without (or with a minimal influence of) surface tension of the solvent [8, 9]. Kistler coined the name “aerogel” because most pores remained in the materials, leading to low-density solids filled with air. Aerogels have then been an important research target because they have unique structural features such as low bulk density, high open porosity, high specific surface area, and high mesopore volume [10, 11]. Among them, silica aerogels typically obtained in the monolithic form offer unique properties, such as visible-light transparency and high thermal insulation ability, because of their finely tuned porous structures. These aerogels thus offer significant potential for various applications such as transparent thermal superinsulation [12], (photo)catalysis [13, 14], biomedical [15], filtration/oil-water separation [16], and flexible devices [17, 18]. However, aerogels with such unique properties are difficult to be manufactured due to extremely low mechanical strength, so supercritical drying (SCD) is needed to prevent pore collapse, irreversible shrinkage, and cracking. Various monolithic aerogels with improved mechanical strength have been investigated to solve these issues and for their practical applications from the viewpoint of controlling materials’ size and shape, nanostructures, and physical properties [19]. For example, cellulose nanofiber aerogels are developed for thermal superinsulation for building windows [20], highly strong aerogels by controlling the structure of nanoscale networks in aramid nanofiber composites [21], boron nitride aerogels with double-paned skeleton network exhibiting negative Poisson’s ratio and negative linear thermal expansion coefficient for thermal superinsulation [22], and the TEMPO-oxidized wood crosslinked with resin for high transparency from ambient pressure drying (APD) [23]. Moreover, there are reports on new manufacturing technology, with precise printed bulk aerogels produced through the production systems using three-dimensional (3D) printers [24].

In our previous studies, we have shown that poly(methylsilsesquioxane) (PMSQ) aerogels containing a methyl silicone framework prepared from methyltrimethoxysilane (MTMS) as the precursor exhibit both high visible-light transparency and excellent mechanical strength/flexibility through the precise controls over their nanostructures [25–28]. The tri-functional polysiloxane network with a methyl group on each silicon enhances spring-back behavior against mechanical compression of the PMSQ aerogels due to the flexible, hydrophobic silicone framework. The unique mechanical properties of the PMSQ gels allow the preparation of aerogels without irreversible shrinkage via APD. More recently, we have prepared PMSQ aerogels with improved bending flexibility employing the two-step acid-base process in the presence of a Pluronic-type nonionic triblock copolymer surfactant [29]. Fibrous skeletons with 4 − 10 nm diameter constructing the 3D porous network allowed even more flexibility in these aerogels. However, these bendable PMSQ gels with fibrous skeletons show incomplete spring-back behavior during APD, resulting in dried gels with higher density.

Although various aerogel-like materials from APD have been studied, many considerations remain to be explored to obtain monolithic aerogels that exhibit physical properties comparable to those from SCD [30]. In particular, changes in the physical properties of PMSQ aerogels associated with the spring-back behavior during APD certainly occur depending on the fine nanostructure and the manufacturing process. Herein, we have succeeded in preparing APD-processed crack-free PMSQ aerogels with glass-like high transparency, consisting of fine nano-size skeletons around 10 nm using Pluronic L64 as surfactant and tetramethylammonium hydroxide (TMAOH) as a gelation catalyst. In the following, starting with practical aspects of preparing highly glasslike transparent PMSQ aerogels, the detailed process of reinforcement of the PMSQ network with MTMS, solvent exchange, and APD is described. Molecular-level structures of the obtained APD aerogels are compared to those of SCD aerogels.

2.1 Materials and equipment

Methyltrimethoxysilane (MTMS) was purchased from Shin-Etsu Chemical Co. Ltd., Japan. Pluronic L64 (EO₁₃PO₃₀EO₁₃, M_w ~ 2900), a triblock copolymer of propylene oxide (PO) and ethylene oxide (EO) chains, was purchased from Sigma-Aldrich Co., LLC, USA. The other reagents and solvents were purchased from Kishida Chemical Co. Ltd., Japan. All the commercially available compounds and solvents were used without further purification. A polystyrene container as the gelation vessel was purchased from Sanplatec Co., Japan.

2.2 Characterizations

Bulk density of the obtained aerogels was determined by measuring the width, height, depth, and weight of cuboid specimens. The spectral data of light transmittance and haze were obtained with a V-670 UV-vis-NIR spectrophotometer (JASCO Co., Japan) equipped with an integrated sphere. The value of total (direct + diffuse) transmittance at 550 nm wavelength was converted to the one in a 10 mm thick sample according to the Lambert-Beer equation and denoted as total transmittance (T₅₅₀).

The porous morphology was observed with a scanning electron microscope (SEM, JSMIT500HR, JEOL Ltd., Japan). Before observation, aerogels were gently crushed and scattered on carbon tape attached to a sample stage. All structural observations were performed with Pt sputtering at 20 mA for 20 s under vacuum using a Pt sputtering apparatus (MC1000, Hitachi High-Technologies Corp., Japan).

The porous structures were analyzed by the nitrogen adsorption-desorption method with BELSORP-mini (MicrotracBEL Corp., Japan). Before the measurement, the samples were degassed at 80°C in vacuo (< 1 Pa) for > 12 h. All the analyses, including those for the Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore size distribution from the adsorption branch, were performed using the software BELMaster.

The chemical structures were investigated by Fourier transform infrared spectroscopy (FTIR) and ²⁹Si cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR). FTIR spectra were measured with a spectrometer IRAffinity-1, Shimadzu Corp., Japan, equipped with a MIRacle attenuated total reflection (ATR) apparatus. ²⁹Si CP/MAS NMR spectra were obtained using an NMR spectrometer equipped with a double-resonance 4-mm MAS probe (Bruker Avance III 800US Plus, Germany, operated at 158.96 MHz). The measurements were carried out at room temperature at the MAS frequency of 12 kHz throughout the study. Hexamethylcyclotrisiloxane was used as external reference material. ¹H 90º pulse width was 4.5 µs. The cross-polarization contact time was 4.5 ms, and the ²⁹Si field strength of 50 kHz was used during the CP period. The pulse delay was 5 s. 1024 scans were accumulated for each measurement. Aerogels were crushed using a mortar before the measurements.

2.3 Fabrication of PMSQ Aerogels with Different Aging Conditions

SOL-GEL SYNTHESIS: MTMS (5 mL) was added to 5 mM acetic acid (5 mL) under stirring in an ice bath, and the mixture was stirred for 12 min. Pluronic L64 (4.2 g) was added to the obtained homogenous solution under vigorous stirring. The solution was sonicated for 10 s for degassing, which was repeated five times and then cooled in an ice bath for 60 min. An aqueous solution of tetramethylammonium hydroxide (0.50 M, 3.0 mL) was added to the cooled solution, which was then stirred for 1 min. The obtained homogeneous solution was immediately transferred to a polystyrene container with a lid to obtain a wet gel within 10 min, which was allowed to stand for 1 d at room temperature.

AGING: The obtained wet gel was heated for aging in an oven at 25, 40, or 60°C for 1 or 3 d in a closed container, in which a solution of methanol (MeOH)/water (1:1, v/v, 5 mL) in an open glass vessel was placed to prevent cracking by possible solvent vaporization from the wet gel.

SOLVENT EXCHANGE: The aged gel was immersed in 2-propanol (IPA)/water (1:1, v/v, 100 mL) at the same temperature as the aging process for 2 d, which was repeated five times in the same manner, while the fifth time was treated at 60°C. The gel was then immersed in IPA (100 mL) for 2 d at 60°C, which was repeated three times. The extracted residues during the solvent exchange were characterized as follows. After each solvent exchange step, the aqueous IPA solution was collected, and pH was measured using a universal pH indicator paper (Macherey-Nagel, Germany). The solution was removed by a rotary evaporator, and the solid residue was dried under vacuum and weighed.

DRYING: The wet gel in IPA was dried from supercritical CO₂ at 80°C and 14 MPa for 10 h to obtain SCD aerogel.

2.4 Modified process for crack-free aerogels by APD and SCD

SOL-GEL SYNTHESIS: PMSQ gels were prepared in the same manner described above in section 2.3.

AGING: A solution of MeOH/water (1:1, v/v, 5 mL) containing 0.1% (v/v) MTMS was poured on the obtained wet gel in the polystyrene container and allowed to stand for 2 d. The gel was heated in an oven programmed to raise the temperature from 30°C to 70°C in 5 h and kept at 70°C for 24 h. The treated gel was removed from the polystyrene container and heated in methanol/water (1:1, 100 mL) at 80°C for 1 d.

SOLVENT EXCHANGE: The aged gel was immersed in MeOH/water (1:1, 100 mL) containing 0.1% (v/v) MTMS at 60°C for 2 d, which was repeated three times in the same manner, while the third time was treated in MeOH/water (3:1, v/v, 100 mL) containing 0.1% (v/v) MTMS. The gel was then immersed in IPA (100 mL) at 60°C for 2 d, repeated three times. Finally, the solvent in the wet gel was exchanged with n-heptane (100 mL) in the same manner and repeated three times.

DRYING: The wet gel swollen with n-heptane was placed in a closed container with pinholes on the lid for slow evaporation of n-heptane. The gel was then heated in an oven at 60°C for 2 weeks to obtain crack-free APD aerogel. The dried gel was finally heated at 120°C for 2 h to remove residual volatiles and mechanical relaxation. In comparison, the wet gel swollen with IPA, obtained at the last step of the solvent exchange with IPA, was dried from supercritical CO₂ in the same condition described in section 2.3 to obtain SCD aerogel.

3.1 Practical preparation of PMSQ aerogels for high transparency

All the PMSQ aerogels were prepared according to the synthetic procedure in Fig. 1. Details of the techniques are described as follows to obtain crack-free aerogels with high transparency. The hydrolysis reaction of MTMS in an aqueous acetic acid was completed within 12 min, resulting in a homogeneous solution of hydrolyzed MTMS and oligomers. Surfactant L64 warmed at 40°C was able to be quickly mixed with the solution compared with the one at room temperature with higher viscosity. This mixture could be homogenized in vigorous stirring, which causes the inclusion of air within the solution. The degassing procedure by sonication was required to remove the air bubbles and dissolved air in the sol to obtain homogeneous, bubble-free wet gels after gelation. Cracking of the wet gel in the following processes of aging and solvent exchange occurred when the degassing was skipped. Microbubbles would have been formed in the gel because of decreased solubility of air in the solution during the sol-gel transition. After mixing with L64, the resultant gel became opaque when the sol was cooled for a long duration of over 1 d (Fig. S1). This result can be explained that the population of cyclic PMSQ oligomers increased in the acidic solution during cooling due to a slow polycondensation reaction [31]. After adding aqueous TMAOH, PMSQ networks with a higher amount of cyclic species formed in the rapid polycondensation in elevated pH, resulting in enhanced phase separation in the network. As a result, until the gelation from hydrolysis of MTMS, each process should be completed within a short period to obtain gels with high visible-light transparency.

Although some bubbles formed after the addition of aqueous TMAOH, they spontaneously moved and disappeared at the free surface of the sol by the timing of gelation. Furthermore, the transparency of the obtained gel was decreased when the sol was sonicated after adding aqueous TMAOH. The sonication treatment after adding aqueous TMAOH may have caused local enhancement of polycondensation, resulting in a broader molecular weight distribution and an opaque, inhomogeneous gel (Fig. S1).

The addition of TMAOH resulted in an immediate increase in the sol temperature from 5 to 15°C. Gelation proceeded quickly after the addition of TMAOH due to the increased pH. When the sol was stirred for more than 1 min after the addition of TMAOH, the resulting gel was not uniform in the container (Fig S2). A uniform gel was obtained when the sol was immediately transferred to the gelation container, which was then allowed to stand without vibration.

After gelation, cracking of the gel tended to occur during aging at 40 or 60°C, solvent exchange, and supercritical drying due to the finer porous structure (skeleton diameter ~ 9.8 nm and pore size ~ 5.3 nm) formed in the presence of L64 as compared to other Pluronic surfactants [29]. The following operations were critical in suppressing the cracking: (1) Aging at least 1 d after gelation, (2) preventing drying of the wet-gel surface by covering it with a mixed solvent of 50% aqueous alcohol during aging, (3) initial solvent exchange with 50% aqueous alcohol, and (4) handling the wet gel in solvent until supercritical drying. At least five times of solvent exchange was required to completely remove the residual surfactant L64 inside the wet gels. The amount of residues eluted in the solution after each solvent exchange is shown in Fig. 2. Residues of eluted components were reduced to zero at the fifth time of the solvent exchange. The accumulated residue approximately equals the total weight of L64 and TMAOH in the starting solution. The pH after the initial solvent exchange was close to 11 − 12, which decreased to 10, 8 − 9, and neutral after the second, third, and fourth times of solvent exchange.

Insoluble precipitates, which were probably polyhedral crystallites derived from MTMS, were eluted from the wet gel during solvent exchange with 50% IPA and precipitated as white powders [32, 33]. The precipitate did not form inside the gel but adhered to the surface of the gel and could be removed by hand; the use of other alcohol solvents, such as MeOH, reduces the formation of the insoluble precipitates.

The solvent in the wet gel was finally replaced with pure IPA and dried from supercritical CO₂ at 80°C and 14 MPa. Even when the wet gel was free from cracks, cracks tended to occur during supercritical drying. Cracking during SCD decreased with increasing aging temperature up to 60°C but was inevitable at 25°C and 40°C. The linear shrinkage after each process from gelation to drying is shown in Table 1 in the aging condition of 25°C for 1 d as an example. While there was almost no shrinkage during gelation, aging, and solvent exchange, there was significant shrinkage during SCD [34]. The drastic shrinkage is mainly because of the additional condensation of remaining Si-OH groups in the PMSQ network under the high-pressure condition of SCD (vide infra).

Table 1

Shrinkage in each step of the PMSQ gel aged at 25°C for 1 d
Process	Shrinkage^a / %
After gelation	0.0
After aging	0.1
After solvent exchange	0.2
After SCD	9.4
^a Shrinkage = 100 − ([gel length on a side]/[vessel inner length on the side]×100)

3.2 Porous structure of PMSQ aerogels and the relation with the aging condition

The aerogel properties of bulk density, visible-light transmittance, and haze with different aging conditions are listed in Table 2 with linear shrinkage values. Bulk density and shrinkage were decreased with the increased aging temperature and longer aging duration. Almost the same bulk density and shrinkage were found in the aging condition of 60°C for 1 and 3 d. The low shrinkage was associated with the improvement of cracking in gels. Almost the same values of bulk density were found in the samples aged at 60°C for 1 d and 3 d.

The precursor MTMS reacts to form PMSQ gels consisting of CH₃SiO_3/2 molecular units via hydrolysis and polycondensation reactions. The theoretical weight of the dried gel can be calculated as 2.35 g from the used amount of MTMS in Eq. 1. The theoretical and actual weights agree well (actual weight was 2.3 − 2.4 g), which means that the wet gel was quantitatively obtained through hydrolysis and polycondensation, which then underwent aging in the basic TMAOH aq. without or negligible elution of the siloxane network. Aging was performed by heating under basic conditions containing ca. 0.13 M TMAOH, accompanied by dissolution-precipitation of the siloxane frameworks based on the Ostwald ripening in silica gel systems [35, 36].

\(\:Theoretical\:weight=5\:{mL}_{MTMS}\times\:0.955\:g\:{mL}^{-1}\times\:\frac{{67.12}_{{{MW\:CH}_{3}SiO}_{1.5}}}{{136.22}_{MW\:MTMS}}=2.35\:g\) (Eq. 1)

Table 2

Physical parameters of PMSQ aerogels prepared in different preparative conditions.
Base^a	Aging	Drying	Bulk density /g cm^− 3	Shrinkage /%^b	T₅₅₀ /%^c	Haze /%
TMAOH	25°C, 1 d	SCD	0.182	9.4	87	6.0
TMAOH	40°C, 1 d	SCD	0.167	5.8	92	3.9
TMAOH	60°C, 1 d	SCD	0.150	1.4	96	1.7
TMAOH	25°C, 3 d	SCD	0.171	6.9	90	3.9
TMAOH	40°C, 3 d	SCD	0.156	3.3	94	2.5
TMAOH	60°C, 3 d	SCD	0.146	1.4	95	2.8
TMAOH	80°C, 1 d	SCD	0.142	1.6	97	3.2
TMAOH	60°C, 1 d	APD	0.331	25	79	8.6
TMAOH	80°C, 1 d	APD	0.148	2.4	95	3.8
LiOH	80°C, 1 d	SCD	0.153	2.0	93	4.1
NaOH	80°C, 1 d	SCD	0.159	2.0	91	4.2
KOH	80°C, 1 d	SCD	0.158	2.0	79	10.4
LiOH	80°C, 1 d	APD	0.160	3.5	92	5.1
NaOH	80°C, 1 d	APD	0.168	3.8	91	4.4
KOH	80°C, 1 d	APD	0.159	3.0	81	9.1
Ethylenediamine	80°C, 1 d	SCD	0.169	4.0	-	-
Hydrazine	80°C, 1 d	SCD	0.181	6.7	-	-
Ammonia	80°C, 1 d	SCD	0.170	5.3	-	-
Ethylenediamine	80°C, 1 d	APD	0.175	4.3	-	-
Hydrazine	80°C, 1 d	APD	0.483	33	-	-
Ammonia	80°C, 1 d	APD	0.404	23	-	-
^a 0.50 M aqueous solution

^b Linear shrinkage calculated from the long side of the gel.

^c Total light transmittance through 10 mm thickness at 550 nm of wavelength.

The tendency of visible-light transmittance is also related to the increasing/prolonged aging temperature and time. The UV-Vis transmittance of the aerogels aged in different conditions is shown in Fig. 3. The total transmittance at 550 nm, T₅₅₀, was maximized to 96% or 95% for the aerogels from aging at 60°C for 1 or 3 d, respectively, which is equivalent to our literature data for PMSQ aerogels [29]; PMSQ aerogels with the finest porous structure and high transparency was obtained using Pluronic L64 as surfactant. The haze values of the aerogels also showed a similar trend to the transmittance. However, the haze value for the aerogel aged at 60°C for 3 d was slightly increased. The surface roughness might have increased due to changes in the wet gel environment caused by solvent evaporation during aging as long as 3 d.

The pore properties of aerogels analyzed by nitrogen adsorption-desorption are shown in Fig. 4a − b and Table 3. The BET surface area decreased in correlation with the increase in aging temperature and duration, and the modal pore size increased as well. The pore size distribution in the range of < 10 nm in diameter was observed at aging temperatures of 25 and 40°C, whereas it was reduced at 60°C. This tendency correlates with partial dissolution and reprecipitation of the nano-sized skeletons by Ostwald ripening, and the strengthened skeletons show smaller shrinkage during drying [35, 36]. In the aging temperature of 25°C or 40°C compared to 60°C, the size distributions of the pores larger than 20 nm in diameter were also observed. This distribution is assumed to be voids caused by the contraction of the nano-sized skeletons during the drying process. Also, this uncontrolled size distribution of pores disrupts the uniformity of the skeletal domains within the bulk body, which is probably one of the reasons for the reduced transparency due to light scattering. Although the pore size can be underestimated because of contraction by the capillary force in the relative pressure range of capillary condensation [37, 38], we believe this tendency is appropriate because the samples aged at higher temperatures, showing larger pore sizes, are less susceptible to this effect. Due to the spring-back property unique to the PMSQ gels, the gels showed a reversible property; there was no shrinkage or cracking after the adsorption-desorption tests.

Micrographs shown in Fig. 4c,d and S3 reveal the nanostructure of prepared aerogels. Contracted skeletons and small pores of around 10 nm as the gaps in-between the skeletons are observed in the PMSQ aerogel aged at 25°C for 1 d as found in Fig. 4c. More distinct skeletons and open pores could be observed by increasing the aging temperature and prolonging the duration as shown in Fig. 4d and S3. This trend correlates with the results of pore size distribution analysis by nitrogen adsorption-desorption.

Table 3

Pore properties obtained by nitrogen adsorption-desorption measurement of PMSQ aerogels prepared in different preparative conditions.
Base	Aging	Drying	BET surface area / m² g^− 1	BJH pore volume / cm³ g^− 1	BJH modal pore diameter / nm
TMAOH	25°C, 1 d	SCD	862	2.57	14
TMAOH	40°C, 1 d	SCD	879	2.61	14
TMAOH	60°C, 1 d	SCD	739	2.44	16
TMAOH	25°C, 3 d	SCD	758	2.04	14
TMAOH	40°C, 3 d	SCD	748	2.41	14
TMAOH	60°C, 3 d	SCD	567	2.15	16
TMAOH	80°C, 1 d	SCD	703	2.84	18
TMAOH	60°C, 1 d	APD	768	2.36	14
TMAOH	80°C, 1 d	APD	724	2.90	16
LiOH	80°C, 1 d	SCD	699	3.11	18
NaOH	80°C, 1 d	SCD	662	2.95	18
KOH	80°C, 1 d	SCD	656	2.96	24
LiOH	80°C, 1 d	APD	720	3.42	18
NaOH	80°C, 1 d	APD	725	3.08	18
KOH	80°C, 1 d	APD	685	3.11	24

The empirical concept of accelerated aging based on the Arrhenius equation for silicone rubber degradation is expected to be applicable to PMSQ aerogels [39, 40]. The acceleration factor of aging per hour at each temperature was calculated, and compared to 25°C, it is 4 and 21 times for 40 and 60°C, respectively. The acceleration factor is based on the Arrhenius equation, K = Aexp(− E/RT), where K is the reaction rate constant, A is the pre-exponential factor, E is the activation energy, R is the gas constant, and K is the absolute temperature. The activation energy E used in the calculation is 72 kJ mol^− 1. The relation of physical properties of aerogels on the corresponding accelerated day calculated by the aging acceleration factors is shown in Fig. 5. The properties of the aerogels showed certain correlations with the accelerated days of the aging condition. Each property shown in Fig. 5 is expected to converge to a certain value with increasing accelerated days in the range of investigation. The approximate plots in Fig. 5 should only be considered as a reference in predicting the aerogel properties. The trend might be reversed in some cases, for example, the light transmittance and haze values after aging at 60°C for 1 and 3 d. Other factors must be considered, such as changes in the aging environment by slow solvent evaporation from the gelation container. However, accelerated aging can be a valuable tool in predicting the physical properties of the resulting aerogels and designing the preparation conditions. These findings have significant implications for preparing and using PMSQ aerogels, providing valuable insights for future research and development in this field.

When the aging conditions became severe, or as the accelerated days became longer, the dried gel finally tended to have lower shrinkage, higher transparency, and fewer cracks (or crack-free) within the range of investigation. After aging and drying, the wet gels consist of PMSQ networks with different concentrations of remaining Si-OH groups, which may be close together by a few nanometers. During the aging process, the condensation reaction between the Si-OH groups is promoted, and their number decreases as discussed below in Fig. 7, which makes the gels more hydrophobic. In the SCD process, wet gels immersed in IPA were exchanged with liquid CO₂ from ambient pressure to 14 MPa at 80°C, reaching the supercritical state. In this process, proximate Si-OH groups further undergo the condensation reaction, which causes additional, irreversible shrinkage on the PMSQ gels [41, 42]. The Si-OH groups should remain abundant, however, especially in gels with milder aging conditions, consistent with the results of remarkable shrinkage and cracking on the gels aged at 25 and 40°C. The PMSQ gels are assumed to be cracked by irreversible shrinkage due to the additional crosslinking during the SCD process. As a result, the aerogel aged at 60°C is considered to be appropriate to improve the cracking problem because of the reduced shrinkage.

3.3 Highly transparent APD aerogels comparable to SCD counterparts

The APD aerogels with high transparency and low shrinkage, comparable with SCD aerogels, have been successfully prepared by optimizing aging, solvent exchange, and drying conditions. The crack-free APD aerogels with high transparency are shown in Fig. 6a. The wet gels were able to be dried at ambient pressure with noticeable spring-back when the final aging temperature was 80°C. In addition, programmed heating with a linear temperature gradient was required. Constant-temperature aging at 80°C or stepwise programmed heating resulted in cracks during the heating. Cracks often occur during aging at temperatures above 60°C. It is assumed that programmed heating with a linear temperature gradient is necessary because the structural changes of the gel associated with a hydrothermal treatment in a basic condition became significant above 60°C. The gels aged at 60°C showed shrinkage with limited spring-back as in Fig. 6a right, and the same was for the gels aged at 70°C.

In the solvent exchange before APD, adding a low concentration of MTMS to the exchanging solution was necessary to obtain low-density, crack-free aerogels. In contrast, only the broken APD aerogel was obtained when no MTMS solution was used in the solvent exchange. Aging conditions in solutions containing a small amount of hydrolyzed alkoxysilanes or oligomers are reported as an effective means for low-density, crack-free aerogels by APD [43–45]. The previous study used a solution of MTMS-derived oligomers hydrolyzed and partially condensed in aqueous urea solution [43]. Still, no clear difference was observed when using unreacted MTMS monomers in this study compared with the previous technique. In any case, it is suggested that a small amount of MTMS reacted on the gel skeletons and reinforced the gel skeletons by linking the remaining Si-OH groups exposed on the skeleton surface similarly to the surface modification. The optimal concentration of MTMS was 0.1 − 0.3% in the solution. At higher concentrations, insoluble precipitations of MTMS-derived condensates, including crystalline polyhedral oligomeric silsesquioxane (POSS), were formed, which disturbed the appearance of the gels [32]. The formation of the insoluble precipitations was significant in IPA, and MeOH was better at preventing the precipitation. The initial concentration of aqueous methanol solution was 50%; cracks tended to form in other initial concentrations. n-Heptane was chosen as the final solvent in the solvent exchange, and the gels were dried from n-heptane because it has a low surface tension (16.5 mN m^− 1 at 60°C) and moderate volatility [46]. When n-pentane and n-hexane were employed, the gels were easier to crack because the drying rate was too fast to control. Since aqueous methanol and n-heptane are not miscible, IPA was employed before exchanging with n-heptane. In addition, exchanging directly from the 50% aqueous methanol to IPA caused cracking, which was improved by using a 75% aqueous methanol solution before IPA.

In APD, warping of the dried gel [47] was a problem, which may be due to preferential drying from specific side(s) and/or differences between the microstructures of the free surface and interfaces contacted with the inner surface of the gelation container. Flat gels were successfully obtained by immersing the gels in 50% aqueous methanol containing 0.1% MTMS for 3 d before heating to 80°C. Shortening the duration resulted in warping of the dried gel, as shown in the APD aerogel from 60°C aging in Fig. 6a bottom-right. These results suggest that the aging based on the Ostwald ripening made the gel more homogeneous regarding the solvent, remaining chemicals, pH, and microstructures in the gel. Additionally, a large-scaled aerogel with 120×120 mm² square with 6 mm thickness by APD was successfully fabricated by carefully following these preparation conditions (Fig. 6b). Thermal conductivity of the large-area aerogel was 15.6 mW m^− 1 K^− 1, which is comparable to SCD aerogels with high transparency T₅₅₀ = 96% prepared in the presence of Pluronic P94 [29], and the other PMSQ aerogels by APD [43]. The successful fabrication of large-area aerogels by APD achieves both high thermal insulation and glasslike transparency, which is suitable for decreasing production costs compared to the process relying on SCD.

The APD aerogel from the gel aged at 80°C showed comparable bulk density, shrinkage, light transmittance, and haze to the SCD aerogels (see Table 2). The SCD aerogel from 80°C aging had the lowest bulk density (0.142 g cm^− 3) and the highest transmittance of T₅₅₀ = 97%, but the haze was 3.2%, which is slightly higher than that of SCD aerogels from 60°C aging (2.8%). In contrast, the APD aerogel from 60°C aging suffered from irreversible shrinkage and increased bulk density. Accordingly, transparency was decreased, as confirmed by the decreased transmittance and increased haze. The pore properties analyzed by the nitrogen adsorption-desorption method shown in Fig. S4 and Table 3 reveal the enlarged pore size distribution around 15–20 nm in diameter for both the APD and SCD aerogels from 80°C aging. Observations of the APD and SCD aerogels under FESEM in Fig. S5 support the pore size distributions obtained from the nitrogen adsorption-desorption analysis. The open pores and the skeletons around 20 nm and 10 nm in diameter, respectively, are recognized in the micrographs. On the other hand, these physical properties of APD aerogels had no clear correlations with the accelerated aging found in the aerogels from 60°C aging or less in the previous section. One of the reasons should come from the different aging conditions of the wet gels.

Spectroscopic analyses revealed only small differences in the molecular structures of APD and SCD aerogels in the ²⁹Si CP/MAS NMR spectra shown in Fig. 7a. The two signals at − 56 and − 68 ppm corresponding to T² and T³, respectively, were found at almost the same chemical shift and intensity in the samples from different aging and drying conditions. The T²/T³ intensity ratio was 0.078, lower than that of the PMSQ xerogels prepared in the presence of CTAC surfactant, which was 0.15 and 0.10 without and with aging, respectively [43]. Although care must be taken that this measurement is semi-quantitative, the diminished T² signal indicates the reduction of the remaining silanol or methoxy groups, and suggests strengthening of the PMSQ network for successful APD in this study. Also, the T² signals were slightly diminished and shifted from − 55.8 to − 56.3 ppm in the samples from 60 to 80°C aging, respectively (inset of Fig. 7a). The upfield shift of T² by − 1 ppm from MeSi-OH group to MeSi-OMe may be explained by the esterification reaction in 50% methanol during aging [48]. The FTIR spectra of APD and SCD aerogels shown in Fig. 7b reveal almost the same absorption bands and, thus, almost the same network structures. However, only a small difference is found in the absorption band by Si-OH vibration around 920 cm^− 1; the band is smaller for the APD and SCD aerogels from 80°C aging due to additional crosslinking and esterification at the higher temperature.

3.4 Alternative gelation process for APD aerogels using other bases

Although highly transparent aerogels can be prepared using TMAOH, its usage is problematic due to its highly toxic nature. Alternative methods to prepare the highly transparent aerogel are welcome to avoid using TMAOH in industrial fabrication. Other bases were therefore studied for the preparation of PMSQ aerogels. Using hydroxides of lithium, sodium, and potassium (these are also strong bases, so they must be handled with care, though), crack-free APD aerogels were similarly prepared (Fig. 8a). The bulk density and shrinkage of these APD aerogels were slightly higher than the corresponding SCD aerogels, and light transmittance and haze of these APD aerogels were comparable to SPD aerogels (see Table 2). The trend of transparency for each APD aerogel prepared with different hydroxides was similar to the corresponding wet gel after gelation (Fig. 8b). Gelation time was about the same for each hydroxide; only gelation using KOH was slightly slower. Transparency is decreased in the order of TMAOH > LiOH > NaOH > KOH, as similarly found in SCD aerogels of poly(vinylsilsesquioxane) [49]. Transparency of the APD aerogel prepared by using LiOH, with a light transmittance T₅₅₀ = 92%, is comparable with that of TMAOH, 95%.

The BET surface area, BJH pore volume, and BJH pore size distribution obtained from nitrogen adsorption-desorption measurement of these APD and SCD aerogels showed a certain trend in the order of LiOH, NaOH, and KOH (Fig. S6, Table 3). The modal pore size for the aerogels using LiOH and NaOH was the same at 18 nm in diameter. In the case of the aerogels prepared using KOH, the pore diameter was slightly increased to 24 nm. A similar trend in pore size and skeleton structure was also found in the FESEM images (Fig S7). A rough correlation can be suggested in the pore size and light transmittance for these aerogels as LiOH ~ NaOH > KOH.

Aerogels were also prepared using amines as base catalysts (Fig. 8c). Compared to alkali hydroxide bases, gelation was slower when amines were employed, which should be due to the lower basicity of amines compared to that of the hydroxides. All the resulting gels were less transparent, which was expected that the slow gelation would lead to the growth of skeletal sizes (see Table 2) due to more enhanced phase separation. The APD aerogels prepared using hydrazine and ammonia showed high shrinkage > 20%. These amines are obviously not suitable for preparing aerogels with high transparency.

Crack-free, monolithic PMSQ aerogels with glasslike transparency have been prepared via APD. The practical keys for the successful preparation of these aerogels are related to controlling the 10-nm-scaled fine structure of the PMSQ skeletons developed in the presence of Pluronic L64 surfactant. In particular, the enlargement of mesopore size over 10 nm and decrease of the remaining Si-OH groups via the Ostwald ripening prevent cracking and irreversible shrinkage during the drying process. Moreover, the PMSQ nanostructures controlled via the aging process show strong correlations with the properties of bulk density, shrinkage, light transmittance, and haze, as well as the resultant porous structures of surface area, pore volume, and pore size distribution. The concept of accelerated aging can be a useful tool in predicting the physical properties of the resulting aerogels and designing the preparation conditions to tailor suitable molecular-level networks and porous structures in the gels. The obtained PMSQ aerogel, aged at 80°C and prepared via APD, is almost equivalent to that from SCD in all physical properties with high transparency (T₅₅₀ > 95%). Due to the hydrophobicity and mechanical flexibility of the PMSQ network, a large-area monolithic aerogel on a large scale (120×120 mm square) was successfully prepared via APD from n-heptane. These new APD aerogels show a similar nature to the previously reported PMSQ aerogels in terms of low thermal conductivity, moisture/water resistance, heat resistance, and lightweight. Furthermore, similar mechanical strength and bending flexibility would be expected.

We have also confirmed that the PMSQ aerogels with similar properties can be prepared using less toxic bases. A similar process for preparing the APD aerogels is applicable when other bases, such as LiOH, NaOH, and KOH, are employed. The APD aerogel prepared using LiOH as base catalyst shows comparable properties with that the one prepared using TMAOH, showing low bulk density of 0.16 g cm^− 3, high transparency of T₅₅₀ = 92% and high BET surface area of 720 m² g^− 1. Although there are still some issues to be considered, such as reducing bulk density and increasing transparency and mechanical strength/flexibility, these practical studies on the preparation and controlling the nanostructure of PMSQ aerogels would provide valuable insights into applications of aerogels.

APD: ambient pressure drying

SCD: supercritical CO₂ drying

MTMS: methyltrimethoxysilane

TMAOH: trimethylammonium hydroxide

TMA: trimethylammonium

IPA: 2-propanol

MeOH: methanol

Fundings

This work was financially supported by collaborative research with tiem factory, Inc. (Japan), under the framework of Research and framework of Research and Development on New Energy Technology for Discovering Technology Seeds and Commercializing Developed Technologies (JPNP10020), administrated by New Energy and Industrial Technology Development Organization (NEDO), Japan. K.K. acknowledges supports from PRESTO (Grant Number JPMJPR22Q2), Japan Science and Technology Agency (JST), Japan. H.K. acknowledges JSPS KAKENHI Grant Numbers JP20H05840 (Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton”).

Competing interests

All the authors have no relevant financial or non-financial interests to disclose.

Author contributions

RM worked on the entire experiment and wrote the manuscript. RU worked on the FESEM observations and revised the manuscript. KT discussed chemical and porous structure analyses and advised on gel preparations. AM and HK worked on the NMR experiments and related discussions. KK designed the whole study, supervised all the experiments and discussions, and revised and finalized the manuscript. All authors have approved the submission of the manuscript.

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Monolithic poly(methylsilsesquioxane) aerogels with glasslike transparency: From sol-gel synthesis to ambient pressure drying

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Abstract

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Highlights

1 Introduction

2 Experimental

2.1 Materials and equipment

2.2 Characterizations

2.3 Fabrication of PMSQ Aerogels with Different Aging Conditions

2.4 Modified process for crack-free aerogels by APD and SCD

3 Results and Discussion

3.1 Practical preparation of PMSQ aerogels for high transparency

3.2 Porous structure of PMSQ aerogels and the relation with the aging condition

3.3 Highly transparent APD aerogels comparable to SCD counterparts

3.4 Alternative gelation process for APD aerogels using other bases

4. Conclusions

Abbreviations

Declarations

References

Additional Declarations

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